1. Field of the Invention
This invention relates to teletext decoders for receiving, storing and processing teletext information in the form of digitally coded data which represents a recurrent transmission cycle of available teletext pages.
2. Description of the Prior Art
A teletext decoder which is suitable for producing character display signals for a teletext display page includes an acquisition circuit for acquiring digital codes representing teletext information for the page, a page memory for storing the acquired digital codes at individual page memory locations, and a character generator for producing the character display signals in accordance with the stored digital codes.
Conventionally, the character generator includes a character memory in which is stored character information representing the character shapes which are available for display. The character memory is addressed selectively using the digital codes store din the page memory to produce the character display signals. The character information for each character shape is stored in at least one individual character memory location which is addressable by a respective digital code.
Current transmission of teletext information are in broadcast television signals. Such teletext transmissions in the United Kingdom are essentially only in the English language, and a character generator which is used with these transmissions includes a character memory containing the character information for an English language character set. Where teletext transmissions are provided in a country whose national language uses a different character set, a character generator which is used with these latter transmission can include a dedicated character memory containing the character information for a character set for the language concerned.
The character requirements of the major European languages differ only in a few (about 11) national use character options. Therefore, these requirements can be met by a single composite character memory containing the character information for a common character set and for a plurality of national option character sub-sets, each of which sub-sets forms with the common character set a complete language character set for a respective language.
In current teletext transmissions, control information which forms part of the teletext information for a page identifies which language character set should be used to display the page. Where a teletext decoder has a character generator with a character memory containing, as aforesaid, the character information for a common character set and for a plurality of national option character sub-sets, this control information is used in the teletext decoder to select a particular sub-set from those which are available and thereby in effect select a single language character set.
As is known from Applicants GB patent specification No. 2 149 627, a small group of the total number of the possible digital codes representing the teletext information may be allocated in common for identifying different character shapes in several national option character sub-sets, the character information for each sub-set being stored in a respective block of character memory locations. The control information is used to determine which block is to be addressed in respect of acquire digital codes belonging to this small group. These acquire digital codes as stored in the page memory are used to address memory locations in the selected block, code conversion being carried out when these digital codes are read out from the page memory to convert them into the appropriate codes for this addressing as a result of the control information.
The selection of a particular one of several national option character sub-sets by means of the control information has the limitation that it is possible to display characters from only one of the sub-sets on a single page, so that it may not be feasible to display a page in which more than one language is to be used, or languages that require more characters than can be accommodated in one national option character sub-set of given size.
In order to overcome this limitation, a teletext decoder may be adapted to substitute a different code for any one or more of the codes which have been acquired and stored in the page memory in respect of a requested page. A substituted code can be appropriate for addressing a character memory location in any of the blocks containing the character information for the sub-sets. To achieve this digital code substitution in the page memory, extension data packets which form part of the teletext information for a page are received and stored in the page memory along with the basic teletext information for a requested page. An extension data packet comprises a number of groups of ancillary control information, one group for each character code to be substituted. Each group comprises three items of data. The first item identifies a character position in the page, the second item specifies the change to be effected, and the third item specifies the character to which the change is to be applied. The teletext decoder determines from the second and third items the digital code which is used to address the character memory location containing the character information for the "new" character shape concerned. This digital code is then substituted for that originally stored in the page memory location which corresponds to the identified character position in the page.
It is a common practice to arrange teletext decoders to operate in such a manner as to acquire the digital codes for a requested page repeatedly from the recurrent cycle of transmitted teletext pages for as long as the page is being requested, and to refresh the contents of the page memory by over-writing the previous contents each time these digital codes are acquired. With this mode of operation, a page memory location in which digital code substitution is to occur, as aforesaid, will have the substituted digital code over-written by the original digital code each time the page memory contents are refreshed. Typically, the recurrent cycle of transmitted teletext pages may be repeated every 20 seconds. As a consequence, in a displayed page the character shape identified by the original digital code is displayed momentarily each time the page memory contents are refreshed until the substitution of the original digital code by digital code for the "new" character shape is effected. This situation therefore gives rise to an undesirable flicker at the character position concerned in the displayed page.